Field of the Invention
The invention relates to a device for adsorbing at least one component of an exhaust gas, having an adsorber through which an exhaust gas can flow. Such adsorbers are used, in particular, in an exhaust system of an internal-combustion engine of a vehicle.
Various concepts for limiting emissions are currently being pursued in order to satisfy increasingly stringent exhaust restrictions throughout the world. The emissions behavior of an exhaust system after a cold start of an internal-combustion engine is at the forefront of those developments. The emissions behavior after a cold start is assessed on the basis of various driving cycles such as, for example, on the basis of the US-FTP 75 test cycle. A driving curve of the FTP 75 test cycle is composed of three speed profiles which have been measured on the streets of Los Angeles, Calif. during the morning rush hour. The FTP 75 test cycle is described, for example, in a book entitled “Kraftfahrtechnischen Taschenbuch” [Automotive Handbook], of Robert Bosch GmbH (22nd edition, Düsseldorf, Germany: VDI-Verlag, 1995).
Catalytic converters are used to clean the exhaust gas. The catalytic converters have a catalytically active surface which, above a defined temperature (approximately 250° C.), chemically converts the pollutants in the exhaust gas (carbon monoxide, hydrocarbons, nitrogen oxides). A time until that light-off temperature is reached can be shortened, for example, by the use of an electrically heatable catalytic converter. However, the increasingly stringent emission restrictions only require very low emissions of hydrocarbons to take place even immediately after a cold start. In order to achieve that, it is suitable to use an adsorber which is responsible for storing hydrocarbon molecules that are produced during the cold start.
Adsorbers having zeolite as a molecular sieve are known. Zeolites are microporous aluminosilicates (aluminum/silicon crystals) with the property of being able to take up certain molecules. The zeolite structure includes a tetrahedral framework with channels and/or cavities, the geometry of which characterizes a zeolite. During the cold-start phase of driving, the hydrocarbons are taken up in the porous structure of the adsorber (adsorption phase). The strength of bonding of the hydrocarbons to the adsorber is dependent on the type of hydrocarbons and the structure of the storage medium. The hydrocarbons stored in the adsorber are desorbed (desorption phase) as the operating time increases and the exhaust-gas temperature rises. That takes place at a temperature of between 100 and 200° C.
It is also known for an adsorber to be disposed in an exhaust system, in which a catalytic converter is disposed downstream of the adsorber. The adsorber constitutes a thermal mass which extracts heat from the exhaust gas. Consequently, when the exhaust gas flows through the downstream catalytic converter, it has a lower thermal energy, thus slowing the heating of the catalytic converter and therefore also delaying the point at which the light-off temperature is reached.